X-ray variations in the inner accretion flow of dwarf novae

¹ Middle East Technical University, Dept. of Physics, Inönü Bulvarı, 06531 Ankara, Turkey
e-mail: solen@astroa.physics.metu.edu.tr
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

Received: 24 April 2012
Accepted: 28 August 2012

Abstract

Aims. We study the inner disk structure of dwarf novae (DNe; i.e., nonmagnetic cataclysmic variables).

Methods. We performed power spectral analysis of the X-ray light curves obtained using the Rossi X-ray Timing Explorer (RXTE) and X-ray Multi-mirror Mission (XMM-Newton) data. We fit the power spectra with a simple model that describes variability as a result of matter fluctuations. In addition, we applied cross-correlation analysis of simultaneous UV and X-ray light curves using the XMM-Newton data to determine time lags between the different wavelength data.

Results. For five DN systems, SS Cyg, VW Hyi, RU Peg, WW Cet, and T Leo we show that the UV and X-ray power spectra of their time variable light curves are similar in quiescence. All of them show a break in their power spectra, which in the framework of the model of propagating fluctuations indicates inner disk truncation. We derive the inner disk radii for these systems in a range of (10−3) × 10⁹ cm. We analyze the RXTE data of SS Cyg in outburst and compare it with the power spectra, obtained during the period of quiescence. We show that during the outburst the disk moves towards the white dwarf and recedes as the outburst declines. We calculate the correlation between the simultaneous UV and X-ray light curves of the five DN studied in this work, using the XMM-Newton data obtained in the quiescence and find X-ray time lags of 96−181 s. This can be explained by the travel time of matter from a truncated inner disk to the white dwarf surface.

Conclusions. We suggest that, in general, DN may have truncated accretion disks in quiescence, which can also explain the UV and X-ray delays in the outburst stage and that the accretion may occur through coronal flows in the disk (e.g., rotating accretion disk coronae). Within the framework of the model of propagating fluctuations, the comparison of the X-ray/UV time lags observed by us in the case of DN systems with those detected for a magnetic intermediate polar allows us to make a rough estimate of the viscosity parameter α ~ 0.25 in the innermost parts of the accretion flow of DN systems.